Carbon And Oxygen Regulation And Pollutant Removal In Constructed Wetland Based On Substrate Amelioration

Constructed wetland(CW)is a tertiary treatment technology with broad ecological prospect.Dissolved oxygen(DO)has a significant impact on the water purification efficiency of CW.The purification of pollutants such as ammonia nitrogen and chemical oxygen demand(COD)mainly depends on the appropriate aerobic environment.The basic oxygenation means in CWs are inflow carry,atmospheric reoxygenation and root release,which are far from meeting the needs of pollutant purification.In CWs,the functional bacteria of denitrification and phosphorus removal are mainly belong to heterotrophic bacteria and need sufficient carbon source to maintain their growth and proliferation.Therefore,the carbon and oxygen regulation of CWs is crucial for the removal of various pollutantsIn order to solve the problems of poor oxygenation efficiency,high energy consumption and insufficient carbon source in the later stage of water treatment in CWs,based on the substrate amelioration,this paper proposed a CW system with carbon and oxygen regulation function.In the basic experiments,part of the substrate was placed in persistent non-liquid immersion state(i.e.,unsaturated state),and the baffle was set in the constructed to guide water,both of them would realize the oxygen regulation The effects of carbon and oxygen regulation were enhanced by adding microfiber and slow-release carbon source in the unsaturated zone.To investigate the effect of substrate amelioration on pollutant removal in CW,the methods of water quality detection,tracer experiment,microelectrode,quantitative polymerase chain reaction,high-throughput sequencing,scanning electron microscopy were used in the whole experiments.The main conclusions are as follows(1)Partially unsaturated CW contains the unsaturated zone in solid-liquid-gas coexistence status,it showed the characteristics of high efficiency and low energy consumption in this study.The oxygen uptake of partially unsaturated CW was 336±44 g m-3 d-1,which was 2.5 times than aerated CW.The daily energy consumption of partially unsaturated C W was 1.1 × 10-4 kwh,which only accounted for 3.3%of aerated CW(3.3 × 10-3 kwh)(2)Partially unsaturated CW showed a lasting and orderly oxygen regulation effect along the pathway.At the end of unsaturated zone,the DO content was 5.08 ± 0.54 mg/L.The obvious removal rate of COD(85±6%)was observed,which was similar to aerated CW(83 ± 6%).The removal rate of ammonia nitrogen was 46.2±11.9%,which up to 11 times that of unaerated CW(3.8±1.9%).In saturated zone,the lowest DO content along the pathway was 0.05 ± 0.01 mg/L,however,the insufficient C/N led to unobvious removal of nitrate in the saturated zone(3)Adding fiber to the unsaturated zone enhanced the effect of oxygen regulation Adding fiber extended the actual hydraulic retention time of the unsaturated zone by 2.2 h,and the DO content(3.39±1.03 mg/L)at the end of unsaturated zone was about 4 times that of the control group(0.92±0.46 mg/L).The microbal abundance in unsaturated zone changed from 8.6 ×1010 copies/g Substrate to 1.05 ±1011 copies/g Substrate.So as to achieve a high NH4+-N removal rate(higher than 97.0%)and to further purify the wastewater.(4)Adding slow-release carbon source to the saturated zone effectively improved the nitrate removal rate and showed a significant effect of carbon regulation.After adding slow-release carbon source,the adjustment of the carbon source structure caused the change of the community structure,and the dominant bacteria in the saturated zone changed from Gammaproteobacteria to Betaproteobacteria.The removal rate of total nitrogen(TN)increased from 20.6 ± 4.0%to 90.4 ± 2.7%,the removal effect of TN was significantly improved(5)The enrichment of ammonia-oxidising bacteria(amoA/Bacterial 16S rRNA)in partially unsaturated CW was 1.43(?),which higher than the two controls(0.27(?)and 1.24(?)).There were a large number of nitrifying and denitrifying bacteria in the unsaturated zone,and the rate of nitrogen conversion is mainly shown as r Deni-NO3-(7.92 ± 0.36 mg N Lsub-1 h-1 L-1)>r AOB(7.05±0.23 mg N Lsub-1 h-1 L-1)>rNOB(4.00 ± 0.28 mg N Lsub-1 h-1 L-1)r Deni-NO2-(3.85± 0.01 mg N Lsub-1 h-1 L-1).